Known in the art is an internal combustion engine arranging an NOx selective reduction catalyst in an engine exhaust passage, feeding an urea aqueous solution stored in an urea aqueous solution tank to the NOx selective reduction catalyst, and using the ammonia produced from the urea aqueous solution to selectively reduce the NOx contained in the exhaust gas, in which internal combustion engine a urea aqueous solution concentration sensor is arranged in the urea aqueous solution tank for detecting an abnormality of the urea aqueous solution (for example, see Japanese Patent Publication (A) No. 2005-83223).
However, this urea aqueous solution concentration sensor is expensive. It is desirable to use another more inexpensive method.